Architectural Design

prunelimitΔίκτυα και Επικοινωνίες

23 Οκτ 2013 (πριν από 4 χρόνια και 8 μήνες)

127 εμφανίσεις

Architectural Design

©Ian Sommerville 2006


To introduce architectural design and to discuss its

To explain the architectural design decisions that have
to be made

To introduce three complementary architectural styles
covering organization, decomposition and control

To discuss reference architectures are used to
communicate and compare architectures

Topics covered


Architectural design decisions


System organization


Decomposition styles


Control styles


Reference architectures

Software architecture

The design process for
identifying the sub
systems making up a system

and the
control and
communication of the framework for sub

architectural design

The output of this design process is a description
of the

software architecture.

Architectural design

An early stage of the system design process.

Represents the link between specification and
design processes.

Often carried out in parallel with some
specification activities.

It involves identifying major system components
and their communications.

Advantages of Design & Document
Software architecture

Stakeholder communication

Architecture may be used as a focus of discussion by system

System analysis

Means that analysis to determine if the system can meet its
functional requirements.

scale reuse

The architecture may be reusable across a range of systems.

Architecture and system characteristics


Localize critical operations in a small number of sub
system and minimize
communications among them. Means use large
grain components rather than fine


Use a layered architecture with critical assets in the inner layers.


Localize safety
critical features in a small number of sub


Include redundant components and mechanisms for fault tolerance.


If it needed, use fine
grain, replaceable components, avoid share data structure

Architectural conflicts

Using large
grain components improves
performance but reduces maintainability.

Introducing redundant data improves availability
but makes security more difficult.

Localizing safety
related features usually means
more communication so degraded performance.

System structuring

Concerned with decomposing the system into
interacting sub

The architectural design is normally expressed
as a block diagram presenting an overview of the
system structure.

More specific models showing how sub
share data, are distributed and interface with
each other may also be developed.

Packing robot control system


Box and line diagrams

Very abstract

they do not show the nature of
component relationships nor the externally
visible properties of the sub

However, useful for communication with
stakeholders and for project planning.


Architectural design decisions

Architectural design is a creative process so the
process differs depending on the type of system
being developed.

However, a number of common decisions span
all design processes.

Architectural design decisions

Is there a generic application architecture that can be
used as a template?

How will the system be distributed across processors?

What architectural styles are appropriate for the

What approach will be used to structure the system?

How will the system be decomposed into modules?

What control strategy should be used?

How will the architectural design be evaluated?

How should the architecture be documented?

Architecture reuse

Systems in the same domain often have similar
architectures that reflect domain concepts.

Application product lines are built around a core
architecture with variants that satisfy particular
customer requirements.

Architectural styles

The architectural model of a system may
conform to a generic architectural model or style.

An awareness of these styles can simplify the
problem of defining system architectures.

However, most large systems are heterogeneous
and do not follow a single architectural style.

Architectural models

Used to document an architectural design.

Static structural model that shows the major system

Dynamic process model that shows the process
structure of the system.

Interface model that defines sub
system interfaces.

Relationships model such as a data
flow model that
shows sub
system relationships.

Distribution model that shows how sub
systems are
distributed across computers.

Architectural models

Different architectural models may be produced
during the design process

Each model presents different perspectives on
the architecture


System Organization

Reflects the basic strategy that is used to
structure a system.

Three organizational styles are widely used:

2.1 A shared data repository style;

2.2 Client Server style;

2.3 An abstract machine or layered style.

2.1 The repository model

systems must exchange data. This may be
done in two ways:

Shared data is held in a central database or
repository and may be accessed by all sub

Each sub
system maintains its own database and
passes data explicitly to other sub

When large amounts of data are to be shared,
the repository model of sharing DB is most
commonly used.

CASE toolset architecture


Repository model characteristics


Efficient way to share large amounts of data; no data transfer

systems need not be concerned with how data is
produced Centralized management e.g. backup, security, etc.

Sharing model is published as the repository schema.


systems must agree on a repository data model.
Inevitably a compromise;

Data evolution is difficult and expensive;

No scope for specific management policies;

Difficult to distribute efficiently.

2.2 Client
server model

Distributed system model which shows how data
and processing is distributed across a range of

Set of stand
alone servers which provide specific
services such as printing, data management, etc.

Set of clients which call on these services.

Network which allows clients to access servers.

Film and picture library


server characteristics


Distribution of data is straightforward;

Makes effective use of networked systems. May require
cheaper hardware;

Easy to add new servers or upgrade existing servers.


No shared data model so sub
systems use different data
organization. Data interchange may be inefficient;

Redundant management in each server;

No central register of names and services

it may be hard to
find out what servers and services are available.

2.3 Abstract machine (layered)

Used to model the interfacing of sub

Organizes the system into a set of layers (or abstract
machines) each of which provide a set of services.

Supports the incremental development of sub
in different layers. When a layer interface changes, only
the adjacent layer is affected.

However, often artificial to structure systems in this way.

Version management system


3. Modular decomposition styles

Styles of decomposing sub
systems into

No rigid distinction between system organization
and modular decomposition.

systems and modules

A sub
system is a system in its own right whose
operation is independent of the services provided
by other sub
systems. A sub
system is made of

A module is a system component that provides
services to other components but would not
normally be considered as a separate system.

Modular decomposition

Another structural level where sub
systems are
decomposed into modules.

Two modular decomposition models covered

3.1 An object model

where the system is decomposed into
interacting object;

3.2 A pipeline or data
flow model

where the system is
decomposed into functional modules which transform inputs
to outputs.

If possible, decisions about concurrency should be
delayed until modules are implemented.

3.1 Object Oriented
decomposition models

Structure the system into a set of loosely coupled
objects with well
defined interfaces.

oriented decomposition is concerned with
identifying object classes, their attributes and

When implemented, objects are created from
these classes and some control model used to
coordinate object operations.

Invoice processing system


Object model advantages

Objects are loosely coupled so their
implementation can be modified without affecting
other objects.

The objects may reflect real
world entities.

OO implementation languages are widely used.

However, object interface changes may cause
problems and complex entities may be hard to
represent as objects.

3.2 Function
oriented pipelining

Functional transformations process their inputs to
produce outputs.

May be referred to as a pipe and filter model (as in
UNIX shell).

Variants of this approach are very common. When
transformations are sequential, this is a batch sequential
model which is extensively used in data processing

Not really suitable for interactive systems.

Invoice processing system


Pipeline model advantages

Supports transformation reuse.

Intuitive organization for stakeholder communication.

Easy to add new transformations.

Relatively simple to implement as either a concurrent or
sequential system.

However, requires a common format for data transfer
along the pipeline and difficult to support event


Control styles

Are concerned with the control flow between sub
systems. Distinct from the system decomposition model.

Centralized control

One sub
system has overall responsibility for control and
starts and stops other sub

based control

Each sub
system can respond to externally generated events
from other sub
systems or the system

s environment.

4.1 Centralized control

A control sub
system takes responsibility for managing
the execution of other sub

return model

down subroutine model where control starts at the top of
a subroutine hierarchy and moves downwards. Applicable to
sequential systems.

Manager model

Applicable to concurrent systems. One system component
controls the stopping, starting and coordination of other
system processes. Can be implemented in sequential
systems as a case statement.

return model

time system control


4.2 Event
driven systems

Driven by externally generated events where the timing
of the event is outwith the control of the sub
which process the event.

Two principal event
driven models

4.2.1 Broadcast models. An event is broadcast to all sub
systems. Any sub
system which can handle the event may
do so;

4.2.2 Interrupt
driven models. Used in real
time systems
where interrupts are detected by an interrupt handler and
passed to some other component for processing.

Other event driven models include spreadsheets and
production systems.

4.2.1 Broadcast model

Effective in integrating sub
systems on different
computers in a network.

systems register an interest in specific events.
When these occur, control is transferred to the sub
system which can handle the event.

Control policy is not embedded in the event and
message handler. Sub
systems decide on events of
interest to them.

However, sub
systems don

t know if or when an event
will be handled.

Selective broadcasting

4.2.2 Interrupt
driven systems

Used in real
time systems where fast response to an
event is essential.

There are known interrupt types with a handler defined
for each type.

Each type is associated with a memory location and a
hardware switch causes transfer to its handler.

Allows fast response but complex to program and
difficult to validate.

driven control


Reference architectures

Architectural models may be specific to some
application domain.

Two types of domain
specific model

Generic models which are abstractions from a number of real
systems and which encapsulate the principal characteristics
of these systems. Covered later.

Reference models which are more abstract, idealised model.
Provide a means of information about that class of system
and of comparing different architectures.

Generic models are usually bottom
up models;
Reference models are top
down models.

Reference architectures

Reference models are derived from a study of the
application domain rather than from existing systems.

May be used as a basis for system implementation or to
compare different systems. It acts as a standard against
which systems can be evaluated.

OSI model is a layered model for communication

OSI reference model

CASE Environment reference model


aided software engineering

Data repository services

Storage and management of data items.

Data integration services

Managing groups of entities.

Task management services

Definition and inaction of process models.

Messaging services

tool and tool
environment communication.

User interface services

User interface development.

The ECMA reference model

Key points

The software architecture is the fundamental framework
for structuring the system.

Architectural design decisions include decisions on the
application architecture, the distribution and the
architectural styles to be used.

Different architectural models such as a structural
model, a control model and a decomposition model may
be developed.

System organizational models include repository
models, client
server models and abstract machine

Key points

Modular decomposition models include object
models and pipelining models.

Control models include centralised control and
driven models.

Reference architectures may be used to
communicate domain
specific architectures and
to assess and compare architectural designs.